Known examples of a drive assist device include a parking assist device that assists the parking of a vehicle with use of images of the periphery of the vehicle captured by a vehicle-mounted camera, and a vehicle periphery monitoring device that alerts the driver upon detecting obstacles or the like in the periphery of the vehicle. JP 2007-237857A (Patent Document 1) discloses a parking assistance device that has a function of determining whether it is possible to secure sufficient space to open a swinging driver's seat door, passenger seat door, back door (rear gate), or the like when parking. A vehicle equipped with the parking assistance device has radar (front sensor and rear sensor) for detecting an object in the periphery of the vehicle and measuring the distance to the object. The space necessary for opening a door (opening space) or distance from the object necessary for opening a door (opening distance) is then calculated, and the result is displayed as a guidance image. In particular, a warning image is displayed if opening is not possible (e.g., paragraphs 26 to 32, 48 to 52, 61 to 66, and 70 to 72, and FIGS. 7, 8, and 10). Also, JP 2005-313774A (Patent Document 2) discloses a rearview device with which a cross-shaped display indicating a vehicle rearward distance from the outermost edge portion of a back door (tailgate) during opening and closing to the rear end portion of the vehicle is superimposed on an image obtained by capturing images of the area rearward of the vehicle (e.g., paragraphs 21 and 25 and FIGS. 3 and 4).